We will continue our studies on the relationship between "specific blocking factors" (SBF), T suppressor cells, and effector cells in tumor immunity. Our working hypothesis is that SBF, in the form of tumor antigen and antigen-antibody complexes, inhibit cell-mediated anti-tumor immunity by activating T suppressor cells which then prevent the establishment of proper T cell help. A third SBF, a protein with a molecular weight of 56,000, is believed to be a T suppressor cell product whose role in suppression is being investigated. We have recently established T-T hybridomas, using T cells from BALB/c mice with MCA-induced sarcomas and hybridizing them with cells from the BW5147 AKR thymoma line. We have shown that a clone of such a hybridoma forms a specific suppressor factor that is active in vivo and in vitro. We are expanding this work by trying to establish suppressor factor producing hybridomas in the MSV sarcoma system, working with an A x BALB/c F1 hybrid mouse. The suppressor factors will be characterized with respect to expression of IJ antigens and the presence of idiotypic determinants. Attempts are being made to develop radioimmunoassays for the different types of SBF.